hey there ! pistils are not a good indicator of maturity. Basically the only reason pistils turn brown is because their membrane gets pierced by the wind, temperature, humidity or contact and the oxigen in the air makes the inside of the pistils rust. that's why they have that color actually. So basically different genetics and different environments have different pistil oxidization schedules. some genetics stay white until the end some get brown early. sometimes it's the grow room fan that will make it happen also. Anyway after you chop they will all turn brown so dont worry. Just harvest based on trichome maturity that's the best you can do. Otherwise you just have to use your intuition. Hope this helps ! 🚀